1. Field of the Invention
Generally, the present disclosure relates to the field of fabricating microstructures, such as integrated circuits, and, more particularly, to the handling of substrates in process tools, such as cluster tools, used for the fabrication of semiconductor devices or other micro structures.
2. Description of the Related Art
Today's global market forces manufacturers of mass products to offer high quality products at a low price. It is thus important to improve yield and process efficiency to minimize production costs. This holds especially true in the field of microstructure fabrication, for instance, for manufacturing semiconductor devices, since, in this field, it is essential to combine cutting-edge technology with mass production techniques. It is, therefore, the goal of manufacturers of semiconductors, or generally of microstructures, to reduce the consumption of raw materials and consumables while at the same time improve process tool utilization. The latter aspect is especially important since, in modern semiconductor facilities, equipment is required which is extremely cost intensive and represents the dominant part of the total production costs. At the same time, the process tools of the semiconductor facility have to be replaced more frequently compared to most other technical fields due to the rapid development of new products and processes, which may also demand correspondingly adapted process tools.
Integrated circuits are typically manufactured in automated or semi-automated facilities, thereby passing through a large number of process and metrology steps to complete the device. The number and the type of process steps and metrology steps a semiconductor device has to go through depends on the specifics of the semiconductor device to be fabricated. A usual process flow for an integrated circuit may include deposition processes, such as chemical vapor deposition (CVD), physical vapor deposition (PVD) and the like, in combination with a plurality of photolithography steps to image a circuit pattern for a specific device layer into a resist layer, which is subsequently patterned to form a resist mask for further processes in structuring the device layer under consideration by, for example, etch or implant processes and the like. Thus, layer after layer, a plurality of process steps are performed based on a specific lithographic mask set for the various layers of the specified device. For instance, a sophisticated CPU requires several hundred process steps, each of which has to be carried out within specified process margins so as to fulfill the specifications for the device under consideration. As the majority of the process margins are device-specific, many of the metrology processes and the actual manufacturing processes are specifically designed for the device under consideration and require specific parameter settings at the adequate metrology and process tools.
In a semiconductor facility, a plurality of different product types are usually manufactured at the same time, such as memory chips of different design and storage capacity, CPUs of different design and operating speed and the like, wherein the number of different product types may even reach one hundred and more in production lines for manufacturing ASICs (application specific ICs). Since each of the different product types may require a specific process flow, possibly based on different mask sets for the lithography, specific settings in the various process tools, such as deposition tools, etch tools, implantation tools, chemical mechanical polishing (CMP) tools and the like, may be necessary. Consequently, a plurality of different tool parameter settings and product types may be simultaneously encountered in a manufacturing environment.
Hereinafter, the parameter setting for a specific process in a specified process tool or metrology or inspection tool may commonly be referred to as process recipe or simply as recipe. Thus, a large number of different process recipes, even for the same type of process tools, may be required which have to be applied to the process tools at the time the corresponding product types are to be processed in the respective tools. However, the sequence of process recipes performed in process and metrology tools, or in functionally combined equipment groups, as well as the recipes themselves, may have to be frequently altered due to fast product changes and highly variable processes involved. As a consequence, tool performance, especially in terms of throughput, is a very critical manufacturing parameter as it significantly affects the overall production costs of the individual devices. The progression of throughput over time of individual process and metrology tools, or even certain entities thereof, such as process modules, substrate robot handlers, load ports and the like, may, however, remain unobserved due to the complexity of the manufacturing sequences including a large number of product types and a corresponding large number of processes, which in turn are subjected to frequent recipe changes.
Recently, process tools have become more complex in that a process tool may include a plurality of functional modules or entities, referred to as cluster or cluster tool, which may operate in a parallel and/or sequential manner such that products arriving at the cluster tool may be operated therein in a plurality of process paths, depending on the process recipe and the current tool state. The cluster tool may enable the performance of a sequence of correlated processes, thereby enhancing overall efficiency by, for instance, reducing transport activities within the factory, and/or to increase tool capacity and availability by using several process chambers in parallel for the same process step. In a cluster tool, several modules or entities are typically served by one robot substrate handler, wherein the different process times, due to different recipes and the like, and/or the parallel processing in some of the modules, may result in competitive transport tasks, thereby generating a dynamic, i.e., a time-varying sequence of events. When several transport tasks are pending at a time, then the robot may operate on the basis of a predefined static rule in order to select an appropriate task for attempting to achieve a desired tool performance, such as maximum tool utilization and the like. This rule may prescribe, for example, to choose the substrate having experienced the least number of process steps from all the substrates requesting transport by the robot handler at this time, or to select the transport task having the destination with the highest predefined priority, and the like. In many cases, the transport sequencing rule is preselected, in view of tool utilization, such that the “bottleneck” module, i.e., the process module of the cluster tool having the least process capacity, is served to enable a substantially continuous operation, thereby typically producing high tool utilization as long as substrates are available at the cluster tool.
As previously explained, an important aspect of semiconductor production is the task of maintaining the throughput of each individual process chamber of a process tool at a high level, which requires that, for a given process recipe, the waiting time at the various process chambers are minimized to thereby achieve the highest possible throughput. For example, assuming that sufficient substrates are available for a continuous operation of the process tool, the entirety of substrate handling activities required for exchanging substrates at a process chamber may determine the overall idle time of the process chamber. That is, during the overall operation of the process chamber, four time periods may contribute to the overall idle time of the process chamber: (1) a time period for waiting for unloading the substrate, that is, the time period after processing of the substrate is completed and the substrate is ready for being picked up by a tool internal transport system; (2) a time period for actually unloading the substrate in which respective substrate handling activities are performed to actually remove the substrate from the respective process chamber; (3) a time period in which the process chamber is waiting for the arrival of a further substrate to be processed; and (4) a time period in which substrate handling activities are performed in order to transfer the substrate from the tool internal substrate handling system into the process chamber.
The time periods (2) and (4) are substantially determined by the characteristics of the tool internal substrate handling system, i.e., the respective robot activities are determined by the hardware capabilities of this unit. On the other hand, the time periods (1) and (3) involve transport activities for, for instance, moving the robot to the unload position of the respective process chamber, i.e., this corresponds to the time period (1), while during the time period (3), the respective robot picks up the new substrate to be loaded and moves it to the loading position of the process chamber. Thus, in view of throughput optimization, it would be advantageous to perform the transport activities required during the time periods (1) and (3) in advance, that is, prior to the end of the processing of a substrate in the process chamber under consideration. In this case, a certain type of look-ahead functionality has to be implemented in the control algorithm. For example, conventional strategies are based on a trigger event obtained from the process chamber to allow the initiation of transport activities during the time period (1), thereby enabling the positioning of the respective robot device at the unload position of the process chamber under consideration in order to immediately receive the substrate after the end of the processing. An appropriate trigger event may, for instance, be the movement of support pins used to position the substrate in the process chamber, which always occurs a few seconds prior to unloading the substrate. By employing this strategy, undue idle time of the process chamber with respect to item (1) may be significantly reduced.
With respect to reducing the waiting time according to item (3), the transport capability of the substrate handling system has to provide the ability to concurrently receive at least two substrates, for instance, in the form of a dual blade robot handler, so that a substrate may be buffered on one blade while the other blade may still be used to unload a substrate currently being processed in a respective process chamber. In order to appropriately exploit the capabilities of the substrate handling system and to provide a certain degree of look-ahead functionality for reducing the waiting time according to item (3), the correct substrate has to be buffered in the substrate handling system, wherein typically the time for picking up the substrate from a load port is significantly longer compared to the simple robot movement to position the robot handler at the unload position according to the time period (1). Consequently, the same trigger event used for reducing or avoiding the waiting time according to item (1) may not be appropriate for providing a look-ahead functionality for picking up and buffering a substrate that is to be processed next in the process chamber under consideration. Due to this fact, it is a frequently employed strategy to use a static rule to define the substrate sequencing for this buffering technique. For example, a rule may be implemented (when assuming a process tool has two process chambers one and two) that leads to the following process strategy: “always buffer substrates at number one position that will actually go to chamber two.” However, in the case of such a static rule, both the timing of the buffering and the selection of the substrate to be buffered may be wrong. Consequently, the efficiency of the process tool, in particular if more than one process step is implemented therein, may significantly decrease.
Additionally, in using a static buffering strategy, the usage of non-occupied transport devices, such as a second blade of a dual blade robot handler, may be available for transport of another substrate since currently the designated process chamber may still be busy. In such a case, an alternative transport activity may be scheduled for the non-occupied transport device, however, with a high risk that the process in the designated process chamber may be finished, thereby resulting in additional idle time of the process chamber due to the non-availability of the required transport capability at the time the process is actually completed. For this reason, a respective static rule may conventionally be implemented in order to forbid an additional transfer activity with a non-occupied transport device, such as a free blade of a robot handler when the corresponding blade is designated for a certain process chamber. As a consequence, the overall throughput may significantly depend on the type and the number of static rules controlling the transport activities of a tool internal substrate handling system, while nevertheless resulting in undue idle times and reduced flexibility.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.